Steel-making process employing an auxiliary composition

ABSTRACT

A COMPOSITION FOR CHARGING TO A METALLURGICAL FURNACE, SUCH AS AN OCYGEN STEEL FURNACE WHICH IS A HEAT- AND FLUXPRODUCING AGENT CONSISTING ESSENTIALLY OF A GRANULAR ADMIXTURE OF FROM 5% TO 95% CALCIUM CARBIDE AND FROM 95% TO 5% SILICON CARBIDE. BOTH COMPONENTS PROVIDE FUEL TO ENABLE THE USE OF HIGHER AMOUNTS OF SCRAP METAL; AND IN ONE EMBODIMENT THE SILICON CARBIDE AND CALCIUM CARBIDE ARE ADMIXED IN A RATIO TO GIVE AN EVENTUAL SLAG OF SUITABLE LIME-SILICA RATIO. THE ADMIXTURE OF SILICON CARBIDE AND CALCIUM CARBIDE IS ADVANTAGEOUSLY ADMIXED WITH A MINOR AMOUNT OF A HYDROCARBON BONDING AGENT WHICH IS AT LEAST SEMI-SOLID AT ORDINARY ROOM TEMPERATURES, SUCH AS TAR OR PITCH, AND FORMED INTO COMPACTS, E.G. PELLETS OR BRIQUETTES. POWDERED ALUMINUM METAL CAN ALSO BE INCORPORATED.

United States Patent US. CI. 75-51 7 Claims ABSTRACT OF THE DISCLOSURE Acomposition for charging to a metallurgical furnace, such as an oxygensteel furnace which is a heatand fluxproducing agent consistingessentially of a granular admixture of from to 95% calcium carbide andfrom 95 to 5% silicon carbide. Both components provide fuel to enablethe use of higher amounts of scrap metal; and in one embodiment thesilicon carbide and calcium carbide are admixed in a ratio to give aneventual slag of suitable lime-silica ratio. The admixture of siliconcarbide and calcium carbide is advantageously admixed with a minoramount of a hydrocarbon bonding agent which is at least semi-solid atordinary room temperatures, such as tar or pitch, and formed intocompacts, e.g. pellets or briquettes. Powdered aluminum metal can alsobe incorporated.

BACKGROUND OF THE INVENTION This invention relates to an adjuvant orcomposition particularly suited for introduction into an oxygen steelconverter, e.g. such as is operated in the LB process for refining iron;and to a method of operating a steel-making furnace with the aid of suchadjuvant, which enables the use of larger-than-usual amounts of scrapper charge to the furnace.

In the oxygen steel process, as is well known, hot. metal and an amountof scrap metal are charged to a refractorylined converter and there arealso charged thereto slagforming ingredients such as fluorspar, bauxite,lime or limestone or both, and if desired, other ingredients to com binewith oxidized impurities in the iron and form a basic slag. Hitherto,the amount of scrap metal has been necessarily limited in this processto about 25% to 30% of the metallic charge, i.e. the total hot metalplus scrap, in order to maintain the required thermal balance in anautogeneous steel-making operation, that is, where there is no externalheat source. Various means have been proposed for enabling ahigher-than-usual incorporation of scrap in such furnaces. These methodshave included preheating the scrap prior to its charging into thefurnace, charging superheated hot metal thereto or other means, all ofwhich have required separate equipment, separate steps and separatehandling of these very hot components of the basic oxygen furnacecharge. On the other hand, the incorporation of calcium carbide as afuel source for oxygen furnaces has also been proposed but has met withvarious difficulties. For instance, the charging of lumps of calciumcarbide of sufiicient size to avoid blowing out of the furnace hasproved inefiicient, apparently because calcium carbide is of relativelyslow solubility in the hot metal wherein its use has been proposed. Onthe other hand, where a finely divided calcium carbide, in small enoughparticle size to enhance its solubility in the molten metal, has beentried, it has required blowing into the furnace, or blowing beneath thesurface of the metal, by means of a separate lance arrangement, whichagain has been expensive. Calcium carbide is also sensitive to moistureand in contact therewith tends to decompose forming acetylene which is aflammable and explosive gas and calcium by- Ice droxide which iscaustic, so that this carbide must be shielded in storage and handlingfrom attack by water or Water vapor. Also, due to its slow rate ofsolution in the molten iron charge, foaming slags tend to occur whichmask the endpoint of the oxygen steel operation.

According to the present invention, the above disadvantages are overcomeand other advantages are obtained, as will be described in more detailbelow, by incorporating in the basic oxygen furnace feed an admixture ofsilicon carbide and calcium carbide in suitable containers or incompacted form, the calcium carbide being present suitably in particlesizes of less than inch diameter; and silicon carbide being suitablyemployed in coarser sizes, for example in sizes of less than a half-inchin diameter. In such an admixture the calcium carbide is in suchphysical state as to enhance its solubility in the molten metal and toutilize to the greatest extent its fuel value as derived from theoxidation of both the carbon and the calcium components of the compound;and on the other hand, the fuel value of silicon carbide from oxidationof the components thereof is also advantageously utilized. Furthermore,the calcium oxide and the silicon dioxide which are formed by oxidationduring the steel-making process provide slag-forming components duringoperation to the furnace and are compatible with other normal componentsof the molten metal environment.

DETAILED DESCRIPTION OF THE INVENTION In the present invention there isprovided a charge or feed composition for the basic oxygen furnace whichprovides fuel and flux components and which comprises a granularadmixture of, based on the weight of the two carbides, from 5% to ofsilicon carbide and from 95% to 5% calcium carbide. In one suitableembodiment of the present invention there is employed one part ofsilicon carbide and from 1 to 10 parts by weight of calcium carbide; andin an especially advantageous embodiment these components are employedin the normal slagforming ratios, that is about 1 part of siliconcarbide and about 5 parts of calcium carbide, this self-fiuxingadmixture being adapted to form during the oxygen furnace reaction aslag having a 3:1 basicity ratio, i.e. about 3 CaO lSiO Suitably theaverage particle isze of the calcium carbide is less than that of thesilicon carbide.

In one embodiment the admixture of silicon carbide and calcium carbideas described herein is thoroughly intermixed to obtain the uniformmaterial and this is introduced into the oxygen furnace in suitablecontainers, such as steel sheet cans, or combustible containers. Inanother embodiment the silicon carbide and calcium carbide in theproportions described above are thoroughly intermixed and to thisadmixture is added a minor amount, sufficient to bond the particles,based on the total weight of the mix, and suitably from about 2% to 20%,of a water-resistant hydrocarbon bonding agent which is at leastsemi-solid at ordinary room temperature, and is suitably a cokablecarbonaceous composition such as tar or pitch, especially as derivedfrom destructive distillation of coal; and the whole is then formed intocompacts such as pellets or briquettes.

In one method of carrying out the present invention, an admixture ofsilicon carbide and calcium carbide as described above is packaged in asuitable container, e.g. a steel sheet can, which will be destroyedduring the furnace operation and will not provide any substantial amountof deleterious components, and is fed into the basic oxygen converter orfurance along with or subsequently to charging the scrap metal afterwhich hot metal is charged to the furnace and oxygen is introduced byblowing from 3 above as in the ordinary L-D operation. Thereafter theusual fluxes can be added in the known way in amounts to supplement thelime derived from the calcium carbide. The blow continues in the usualway until the flame drops, whereupon the metal is Withdrawn.

In one mode of carrying out the present invention, the desired admixtureof silicon carbide and calcium carbide, for instance, from about 30% toabout 70% calcium carbide in particle sizes less than 4 inch in diameterand from 70% o 30% of silicon carbide of less than /2 inch in diameteris thoroughly and intimately blended and there is intimately admixedwith such admixture tar or pitch in an amount of from 3% to 10% byweight based on the total weight of the batch, and the whole thenbriquetted to form strong compacts. In this manner, the cokablecarbonaceous bonding agent protects the finely divided calcium carbidefrom attack by moisture or water which would result in decompositioninto calcium hydroxide and acetylene gas.

In another and advantageous mode of procedure, hydrocarbon bondingagent, especially coal tar or coal tar pitch, and the silicon carbideparticles are separately heated to 200-300 F., then mixed together andthereafter unheated calcium carbide fines are mixed in, the wholecompacted into shapes and cooled to ambient temperature. The finishedcompacts can also be dipped, coated, sprayed or the like with ahydrocarbon bonding agent of the type described herein and, if desired,with incorporation of a portion of the silicon carbide, to protect thecalcium carbide further from water attack.

The compacts are easily stored and shipped and in furnace operation arecharged into the furnace as described above. The compacts are suitablywell distributed over the cross section of the furnace and hot metal isthen charged thereto, whereupon the compacts are uniformly distributedthroughout the cross section of the charge of hot metal and the carbidecomponents are uniformly available for reaction therein.

In carrying out the oxygen steel-making process using the carbideadjuvant according to the present invention, the amount of scrap whichcan be added is substantially increased above the normal 25% to 30%thereof. The carbides adjuvant is added in an amount to provide fuel tomaintain the steel-tapping temperature at the desired level with theamount of scrap being added. As an example of the use of the compositionaccording to the present invention, a self-fiuxing mixture of 5 parts byweight of calcium carbide of particle sizes passing inch by down and 1part by weight of silicon carbide of particle sizes inch by downprepared as described above is fed to an oxygen steel converter, i.e. anL-D furance, in an amount to provide 40 lbs. calcium carbide and 8 lbs.silicon carbide, per ton of steel produced, being charged to the furnacewith an amount of scrap metal. There is charged in this instance 40% ofscrap metal based on the total weight of the hot metal plus scrapwhereas only 30% scrap would be used herein in the absence of the addedcarbides. Oxygen blowing is carried out with suflicient extra oxygen toreact with added carbides, to produce a heat of steel at the desiredtapping temperature and carbon content. In another mode of using thecomposition according to this invention there are employed 1 part byweight of minus inch calcium carbide and 1 part by weight of siliconcarbide of minus inch particles and in total amounts, respectively, toprovide 17 lbs. of each carbide per ton of steel produced, particularlyin treating a low-silicon hot metal starting charge. In this latterinstance, sufiicient lime is provided in the flux charge to react withthe excess silicon produced by oxidation of the silicon carbide in thefurnace operation over that satisfied by the added Ca carbide and toprovide the proper slag V-ratio.

In the admixture consisting essentially of calcium carbide and siliconcarbide as described hereinabove, there can also be admixed a smallamount up to about 35% by weight of the total admixture of powderedaluminum metal as a fuel and which also undergoes exothermic oxidationduring the oxygen blowing step and is believed to form alumina useful asa slag component. There can be incorporated, alternatively oradditionally, a minor amount of iron ore, millscale, fiuorspar, bauxiteor other desired additive compatible in the oxygen steel-making processor any suitable combination thereof, and the amount added, including anypowdered aluminum metal, will not exceed about 35 of the totaladmixture.

It is an advantage of the persent invention that greatly increased fuelvalues are incorporated in the oxygen furnace charge and that thereforethe amount of scrap which can be charged in one blow is greatlyincreased over the normal amount of scrap charged. The components of thefurnace adjuvants described herein are all compatible with the furnacecharge in the molten metal and provide on the one hand fluxing and slagcomponents, that is lime and silica, and on the other hand gaseouscomponents, carbon oxide gases, which go off with the other furnacegases. The tar or pitch bond or a combustible container adds stillfurther fuel in this operation. It is a great advantage that the carbidematerials can be widely distributed over the molten metal charge and arein such physical state that they do not blow out of the furnace,particularly the calcium carbide which is of relatively slow solubilityrate in the molten metal. By the present invention, the calcium carbideis more readily available for reaction because of its small size and itis retained in the furnace zone where needed for reaction because of thephysical state wherein it is added. It is an advantage over blowingcalcium carbide into a hot metal bath that by the present method noadditional inert carrier gas is required and the calcium carbide is welldistributed through the hot metal. On the other hand, when added bymeans of an inert gas stream, it is fed in at the top of the melt and inone restricted area, being necessarily distributed therefrom by thestirring or turbulence in the metal bath. By the present method andmeans the calcium carbide is well distributed over the bath andsolubilization therein is enhanced. It is believed a further advantagethat the silicon carbide dissolves more readily and thereupon enhancesthe rate of solubility of the calcium carbide in the hot metal. It isbelieved also that this effect may result from increased temperature ofthe hot metal bath because of rapid oxidation of the silicon carbide,which is an exothermic reaction.

Although in the above specific description the carbide adjuvantcomposition has been placed in the converter with the scrap, it will beunderstood that any other desired sequence of charging can be employed.Thus, there can be charged in sequence, scrap, carbides, hot metal, thenfluxes; or scrap, hot metal, carbides and fluxes; or scrap, hot metal,fluxes and carbides. In each type of operation oxygen blowing issuitably started after charging of the hot metal. In this descriptionthe added metallic coolant described has been scrap metal but it is tobe understood that scrap or scrap metal is intended to include othermetallic coolants such as millscale, cold pig iron, iron ore, ore sinteror the like. Percentages herein are by weight unless otherwiseindicated; and mesh sizes are Tyler screen sizes. Aluminum metal cans orcontainers can be used instead of sheet steel cans.

Having now described the invention, what is claimed 1. In a method ofmaking steel wherein molten ferrous metal containing oxidizableimpurities, scrap metal, slagforming agents and fluxes are charged intoa steel-making converter and an oxygen-providing gas is blown onto thetop of said molten metal until tapping temperature and carbon contentare attained, the improvement comprising charging into said converterprior to completion of blowing with oxygen-providing gas, a granularadmixture of 5% to silicon carbide and 95% to 5% calcium carbide, basedon total weight of carbides, and an amount of scrap metal larger thanwould be liquefied by the heat developed by oxidation of said oxidizableimpurities.

2. Method according to claim 1 wherein said molten ferrous metal isdeficient in silicon impurity and there is charged a granular admixtureconsisting essentially of a major portion of silicon carbide and a minorportion of calcium carbide to provide the desired slag volume.

3. Method according to claim 1 wherein said calcium carbide is presentin an amount of about five parts by weight for each one part by weightof silicon carbide.

4. Method according to claim 1 wherein said silicon carbide is ofparticle size passing one-half inch mesh and said calcium carbide is ofparticle sizes passing onequarter inch mesh.

5. Method according to claim 4 wherein the average particle size of thecalcium carbide is less than that of the silicon carbide.

6. Method according to claim 1 wherein said admixture of silicon carbideand calcium carbide has been compacted prior to charging into theconverter.

7. Method according to claim 1 wherein said admixture of silicon carbideand calcium carbide is charged into the converter prior .to commencementof blowing with oxygen-providing gas.

References Cited UNITED STATES PATENTS OTHER REFERENCES Bailey, D. R.,and Onuscheck, J. W., High Scrap Charge in the B.O.F. Utilizing SolidFuel, Blast Furance and Steel Plant, March 1967, pp. 232-237.

L. DEWAYNE RUTLEDGE, Primary Examiner G. K. WHITE, Assistant ExaminerUS. Cl. X.R.

